Methods for PLC to Save Input Points (2026 Update)

Detail

Use a single analog input (4, 20 mA or 0, 10 V) and a resistor ladder to read 4 to 8 binary sensors, on any rack that has a spare AI slot (e.g. 1756-IF8H, 6ES7 331, Q68ADI).

Drop in a digital multiplexer (e.g. CD74HC4067, Omron G7TC) in front of one high-speed counter to share one input across many.

Re-tasking unused outputs as inputs on compact PLCs (1769, MicroLogix 1400, CP1E, FP0R) recovers 4 to 16 points for free , with the scan-order caveat.

Distributed I/O over EtherNet/IP, PROFINET, or CC-Link IE cuts the wiring back to the CPU and lets you add a 1734 POINT block 50 m away instead of a second rack.

Shift-register chains (BELDEN 8790 pattern, conveyor tracking) and series-wired safety groups still beat expanding the panel when the application allows.

For 2026 SLC 500 retrofits, Beckhoff EP6224 IO-Link masters and Wago 750-15xx are the cleanest way to keep 1747-ASB wiring alive without buying another SLC rack.

Three forces are squeezing rack space on brownfield lines this year. First, the post-2024 Allen-Bradley price uplift on 1756-IB16IF and 1756-IH16ISO modules has pushed most U.S. integrators to look at re-distributing I/O rather than buying a second chassis. Second, the Siemens S7-300 (6ES7) family is officially in phase-down , lead times on 6ES7 321-1BL00 SM 321 DI32 modules now stretch past 24 weeks through European stockists, which pushes S7-1500 / ET200MP retrofits onto more projects that originally planned to stay on S7-300. Third, Mitsubishi MELSEC-Q spare-part availability has stabilized, but the iQ-R migration is still adding rack slots because R-series analog cards (Q68ADV(I)) consume more backplane budget than the older Q68ADI they replace. Across all three vendors, the practical answer is the same: get more out of the input points you already have.